A mechanical double-action press machine is normally provided with a regulating mechanism which regulates the press load. This regulating mechanism, as shown in FIG. 1, comprises hydroblank holder 4, protector cylinder 8, pneumatic device 12 and hydraulic device 15.
The hydroblank holder 4 incorporates piston 1 and has an air chamber 2 and an oil chamber 3 defined by the piston 1. The protector cylinder 8 defines an oil chamber 7 at a connection part between a connection screw 5 and an outer slide 6. The pneumatic device 12 includes pneumatic source 9, pressure regulating valve 10 and accumulator 11 and supplies a predetermined pneumatic pressure to oil chamber 2 in hydroblank holder 4. The hydraulic device 15 includes hydraulic source 13 and check valve 14 and supplies hydraulic pressure to oil chamber 3, in hydroblank holder 4, and oil chamber 7, in protector cylinder 8.
When a pneumatic pressure supplied to air chamber 2 in hydroblank holder 4, i.e., the regulator pressure, is set to a high level in the mechanical double-action press machine equipped with such a regulating mechanism, the hydraulic pressure does not operate, the incremental load produced by the pressure application of the press machine is transmitted directly to a press die through outer slide 6 to carry out a mechanical zone forming operation. In this case, the outer load is determined primarily by the die height of outer slide 6.
On the other hand, if the regulator pressure supplied to air chamber 2 is adequately balanced against the hydraulic pressure, the above mentioned incremental load is absorbed by the operation of piston 1 of hydroblank holder 4. Therefore, the load is transmitted to the press die through hydraulic pressure to carry out a hydraulic zone forming operation. In this case, the outer load is determined by the die height of the slide and the regulator pressure.
The values of the die height of the slide or the regulator pressure are determined through the procedure of finding the optimal processing condition beforehand by a test pressing. The values determined in the above procedure are set in the press machine in an actual production line and the press machine presses in accordance with the conditions thus set.
However, there exists an unavoidable mechanical difference between the two press machines. The regulator pressure is set manually by an operator monitoring meters; hence, it is extremely difficult in practice to effect precisely uniform regulation. As a result, a significant number of defects such as surface distortion and cracking are produced by the shifting dynamic characteristics of the outer load varying from the ideal condition. Therefore, to solve this problem, the dies are adjusted by a fitting process before they are brought into the production line to compensate for the machine difference and manual operation error of the regulator pressure.
However, the above method for solving the problem is not appropriate because the process of carefully fitting and adjusting the dies is time consuming; hence, productivity declines.
Moreover, as pneumatic pressure in a plant fluctuates with the state of the operation in each section and is influenced by a pneumatic source, the forming defects can also be produced by deterioration of the predetermined balance between the regulator pneumatic pressure in air chamber 2 and hydraulic pressure in oil chamber 3. Adjusting the dies cannot solve the problems caused by such factors as these, which occur during pressing. Therefore, fitting and adjusting the dies is not appropriate from this standpoint either.